1. Field of the Invention
This invention relates generally to gages for measuring the functional and individual parameters of threaded members. More specifically, the invention relates to an indicating thread gage for measuring the cumulative, functional parameters and the individual parameters of threaded members such as tubular products with tapered threads.
2. Description of the Related Art
Various components used in the oil and gas production industry are provided with threads at their ends to join one to another. These components include line pipe, casing, tubing, couplings, rotary joints, various equipment, and all other connections. The threads are usually tapered and the thread form may be conventional such as 8 round, buttress, or any other thread form used in the petroleum industry, including premium connections.
It is important for proper performance and extended service life of the threaded components that the joints between the various components be strong, durable, and tight. This requires that the threads used to form these joints be fabricated with precision. This is particularly important for tapered threads.
To obtain the high precision necessary for such connections, it is desirable that the thread sizes and configurations for threaded components used in the oil and gas production as well as other applications be standardized for the particular industry so that quality control can be uniformly maintained throughout the industry. In the oil and gas production industry, the American Petroleum Institute (API) has adopted specifications for threading, gaging and thread inspection of components used in the industry. For example, API specifications 5B and 7 set forth standard dimensions for pitch diameters, thread pitch, thread lead, taper, and other parameters for the various types and sizes of threads corresponding to a range of pipe and tubing diameters. These API specifications also set forth the gaging distance for particular threaded products. For example, API Std. 5B and API Spec. 7 specify the gaging distance for pipe and casing to be a certain distance (depending on nominal diameter) from the shoulder or face of the threaded product.
In addition to specifying the standard dimensions for the various thread parameters, the API specification also sets forth a procedure for checking the accuracy of the actual thread parameters of the threaded product as compared to the design dimensions. This procedure, which is universally accepted and used by the petroleum production industry, utilizes a series of ring and plug gages to check the accuracy of the actual thread against the API standard thread for the particular thread configuration and threaded product. This series of gages includes a plug and mating ring for each thread configuration covered by the API specification. These thread configurations vary by diameter, taper, pitch and type of thread, e.g., round or buttress. The number of thread configurations covered by the API specifications is large and the manufacturer desiring to fabricate all of these thread configurations therefore needs a like number of plug and mating ring pairs. This does not include the many non API thread forms now in use.
Further, the API specifications also require the manufacturer to have a set of working gages and to have access to a set of reference master gages. The working gages are used for actually gaging the product threads, while the reference master gages are used primarily to periodically check the accuracy of the working gages and only rarely to check the product threads. Because the rings and plugs are made of relatively expensive material and require substantial machining and grinding to very precise tolerances, a complete set of plugs and mating rings costs hundreds of thousands of dollars. Also, the threads of the rings and plugs eventually wear out from repeated use, necessitating replacement.
The thread parameters of tapered threads, unlike straight threads, are defined at a specific location on the thread. As discussed above, the API specifications list a gaging distance for each size of threaded product. The gaging distance defines a location on the thread a certain distance from an identifiable reference point such as a rotary shoulder, box end, or pin end. With ring or plug gages, the gage is threaded onto the product threads until hand tight engagement is achieved. Then, the so called "stand-off" distance of the ring or plug from the reference point is checked against the stand-off distance defined by the API specification. Standoff is defined as the distance from the reference point to a known point on the gage, usually a lip. The measured stand-off must fall within acceptable limits established by API for a particular threaded product. Thus, this gaging system is not capable of directly measuring individual thread parameters. It only provides a cumulative measurement of the individual parameters (i.e., functional diameter or fit) by a comparison between actual stand-off and reference stand-off.
As a result, the ring and plug gaging system established by API possess certain inherent shortcomings. As stated, rings and plugs do not directly measure individual thread parameters such as pitch diameter, thread pitch, lead, or taper. Ring and plug gages can measure only the functional fit of the threads. And, in fact, ring and plug gages can only accurately measure perfectly formed threads. If the threaded product is slightly oval shaped or slightly triangular shaped, the stand-off measurement obtained by ring and plug gages will not represent the true condition of the threads. Similarly, any error in pitch diameter, thread pitch, lead or taper will yield an erroneous stand-off measurement. Moreover, just because two threaded products pass the API ring and plug gage requirements, that does not mean that the two products will thread together properly. It is known in the industry for products that have passed the API ring and plug requirements not to pass requirements of torque-turn or torque position. In other words, during make-up of the threaded connection, the threaded products will require either too much or too little torque to engage a specified number of threads. Further, the stand-off of ring and plug gages are typically measured to a resolution of about one mil (0.001"). Still further, the accuracy attained by hand tight engagement of the ring or plug may be affected by any of the following: damage or distorted threads, improper thread form, incorrectly machine thread elements, ovality, cleanliness, or excessive lubrication which can affect the accuracy by as much as 0.100" or more. Finally, ring and plug gages are bulky, heavy and expensive.
Despite those shortcomings, ring and plug gages are the only API approved gaging system for measuring the functional fit of tapered threads. In contrast, there are a number of different gaging systems available for measuring both individual parameters and functional parameters of non-tapered or straight threads. For example, ANSI Standard B1.2-1983, entitled "Gages and Gaging for Unified Inch Screw Threads" provides for at least the following different gage systems for straight threads: split or solid threaded rings, thread snap gages, and indicating thread gages with either thread segments or thread rolls.
Indicating thread gages offer the advantages of offering a direct read-out of the thread parameter being measured, allowing a number of different sized threaded products to be measured with a single indicating thread gage, and the versatility to measure both functional thread parameters and individual thread parameters.
Others have attempted to provide an indicating gage using thread rolls or thread segments for tapered threads. For example, the Johnson Gage Company offers a "Tri-Roll Type" indicating thread comparator. However, this indicating gage is deficient insofar as it is incapable of gaging a threaded product at more than one circumferential position. Further, the Johnson gage is incapable of checking for ovality, or gaging the individual parameters of the thread form. Still further, the Johnson gage has a limited range of applicability to various sizes of threaded products, and the Johnson gage requires the use of an expensive and bulky ground reference master for calibration.
Thus, it has long been desired to develop an indicating gage capable of measuring both functional and individual thread parameters, having a wide range of applicability, and capable of replacing the API ring and plug gaging system.